This invention relates to a smart card for a user station used in a mobile communications system, such as a subscriber identity module (SIM) as used in a GSM (Global System for Mobile communications) digital cellular radio system, to a mobile station including such a smart card, and to a method of transmitting data items to and from such a smart card.
In a known conventional GSM system, each mobile station, such as a mobile telephone handset, is provided with a SIM, herein also referred to as a smart card, which is inserted into the mobile station in order to allow the mobile station to receive service in a GSM network.
A typical SIM includes a microprocessor, memory elements including a permanent memory (e.g. ROM), a non-volatile rewritable memory (e.g. EEPROM) and a volatile rewritable memory (e.g. RAM), and contacts for forming the data transfer interface between the SIM and the mobile station.
The GSM standard also specifies a command set for use by a mobile station to access data records in the SIM and retrieve and write data from and to the SIM. Such data records may be permanent data records, such as a record of the international mobile subscriber identity (IMSI) whereby the mobile subscriber is recognised by a mobile communications network.
Other data records are modifiable, either at the initiative of the mobile station (such as the data record holding the current location area information (LAI) for the mobile station) or at the initiative of the mobile subscriber (such as an abbreviated dialling number (ADN) record which is used to allow the subscriber to automatically initiate calling of the stored dialling number by selection of the associated dialling number identifier when displayed by the mobile station).
Other modifiable data records include a short message data record, which stores data relating to one or more short messages in alphanumeric characters received by the mobile station, for future retrieval by the subscriber.
For more details of the records which may be stored in a SIM, reference should be made to GSM Technical Specification 11.11 (ETS 300 608).
The memory capacity of the SIM is necessarily limited, primarily due to cost considerations.
Accordingly, the data records referred to above are limited in size and number in accordance with the memory capacity of the SIM, which can be a drawback. For example, in the case of ADN records, it is conventional to allocate sufficient memory capacity dedicated to an ADN list such that up to 90 ADN records can be provided. Some subscribers however require a greater ADN storage capacity. This is evidenced by the fact that ADN storage capacity is used as a positive marketing point by suppliers who are able to provide SIM""s having relatively large ADN storage capacities. However, with a fixed amount of total memory capacity available, an increase in the memory capacity allocated to ADN records must be compensated for by a reduction in the memory capacity allocated to other data records.
Further information relating to known subscriber identity modules used in GSM may be found in Farrugia A J et Al.: xe2x80x9cSmard Card Technology Applied to the Future European Cellular Telephone on the digital D-Networkxe2x80x9d, selected papers from the Second International Smart Card 2000 Conference, Oct. 4-6 1989, Amsterdam, NL, 1991, pages 93-107.
GB-A-2251357 describes a radio telephone terminal which includes a terminal unit having a transmitter-receiver and an external device. The external device has a storage unit for storing subscriber information, a recaller unit for selectively recalling the stored information. Information may be recalled from the external device to the radio telephone unit, so that it is possible to originate a call from the terminal unit using a subscriber telephone number stored in the external device without the need for a dialling operation on the terminal unit. Thus, subscriber information stored in the terminal unit may be augmented by subscriber information stored in the external device.
U.S. Pat. No. 4,875,230 describes a cellular telephone which includes logic circuitry allowing keystroke sequences to be stored and recalled for activation using a short form recall command. It does not describe the use or a smart card.
The present invention in one aspect provides a smart card for a mobile station used in a mobile communications system, said smart card including:
a memory for storing data items communicated via an interface between said card and said mobile station; and
data item converting means,
said converting means being for receiving a data item transmitted via said interface in a standardised data coding format, for converting said data items to a compressed data coding format and for storing said data item in said memory in said compressed data coding format, and
said converting means being for receiving said data item from said memory in said compressed data coding format, for decompressing said data item to said standardised data coding format and for transmitting said data item via said interface in said standardised data coding format.
Thus, even if a standardised data coding format for a data item transmitted via the smart card/mobile station interface is not optimal for storage, by use of the present invention the data item can still be stored in the smart card in a compressed data coding format, without rendering the smart card incompatible with the chosen standard.
The data coding format conversion can be implemented invisibly to the mobile station. That is to say, the communication of data items via the interface between the smart card and the mobile station can be implemented in the data coding format required by the chosen standard, irrespective of the data coding format used for stored items.
Thus, the smart card can remain compatible with all mobile stations complying with the chosen standard.
In some cases, data items are required to conform to a standardised fixed length data coding format when communicated to or from the mobile station. The data item converting means then preferably converts data items from said fixed length data coding format to a variable length data coding format. More efficient data coding can thereby be achieved. For example, in the case of ADN records, the use of a variable length coding format for storage of ADN data items can increase the amount of ADN records available, compared to the conventional storage format, by over 50%. Thus, the number of ADN records available can be increased to over 100, without increasing the memory allocated to the ADN list.
The data items may comprise a variable Quantity of non-padding character data (i.e. data relating to useful information), and when in the standardised data coding format a variable quantity of padding data (i.e. data of which the contents are ignored). The data item converting means is preferably operable to reduce the amount of padding data when converting the data items to the different data coding format. Data items can then be stored in a more compact coding format in a given allocation of memory capacity.
In a further aspect, the invention provides a method of writing a data item to, and transmitting a data item from, a smart card for a mobile station used in a mobile communications system, said card including a memory for storing data items communicated via an interface between said card and said mobile station, wherein said method comprises:
receiving a data item transmitted via said interface in a standardised data coding format, converting said data items to a compressed data coding format and storing said data item in said memory in said compressed data coding format; and
receiving said data item from said memory in said compressed data coding format, decompressing said data item to said standardised data coding format and transmitting said data item via said interface in said standardised data coding format.